Polyurethane topcoat, also abbreviated as PU topcoat is widely used for coating industry. It is known that polyurethane topcoat after curing may provide a coating having good adhesion to its underlying coating such as epoxy primer coating and having good weatherability, corrosion resistance and high gloss. Therefore, polyurethane topcoat is basically suitable for all application fields of industrial paint and thus its need may reach up to one or more million tons per year.
Taking into account actual use, gloss of polyurethane topcoat is one of factors determining its application field. At present, the prior polyurethane topcoat after cured usually produce a coating exhibiting a gloss of at most 70% at 20°. Apparently, this kind of PU topcoat is not sufficient to achieve the requirement to advanced paints such as vehicle finish paint. In practice, PU topcoat is desired to exhibit a gloss of at least 80% at 20° to ensure that is may be suitable for applying metal substrate. It is known that flatting performance of PU topcoat plays an important role in achieving its decorative effect, especially gloss. The PU topcoat having good flatting performance may produce a coating having high gloss, high DOI and fullness. In contrast, the PU topcoat having poor flatting performance after film-forming will form undesired coating with peel structure exhibiting obvious ripple.
There is still a need for polyurethane topcoat having excellent flatting performance and producing a coating exhibiting high gloss and high DOI.